My opinion on alt policy is really individualized to the game.
Is it a small game, with a single competative "sphere" where it is highly likely to have all PCs or many of them constantly in each other's space?
Then I am heavily in favor of one PC per player.
Is it a small game, cooperative, where it is highly likely to have all PCs or many of them constantly in each other's space?
Then I am okay with alts, but would prefer 2-3 max per player, with restrictions on class of PC to minimize alt crossover (either of the my PCs are helping each other out, or I will make 3 PCs that all center around this other player's 1 PC variety).
Is it a large game, with plentiful staff, where there is in theory ample room for PCs to be way spread out with no interaction at all? Sky's the limit.
With the following caveats for the latter two:
COI must be defined and rigidly enforced. First time catching bad behavior, you get to pick on alt and retire all the rest. Poor player behavior is to be managed. (As in, if you have staff who do not want to run scenes at all because Asshole has to be in EVERY SINGLE SCENE with an alt and behaves like...themselves...then you need to punt asshole, or set limits on how many staff run scenes per week that any player can attend, or some other management of the actual problem person.)